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Abstract
In this work, periodic rectangular arrays were fabricated on quartz substrates using the femtosecond laser ablation technique, on which inorganic cesium lead bromide thin films were grown using the spin coating method. Enhanced photoluminescence emission was investigated using a homebuilt confocal microscope, and increased light absorption due to the engineered structures was also measured. High-performance amplified spontaneous emission with typical narrow lasing emission peaks excited using a nanosecond laser centered at 266 nm was obtained. This work provides a method to modify the performance of optoelectrical devices, which helps develop light-emitting diodes, photodetectors, solar cells, and lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Halide lead perovskites (HLP) have gained significant attention for their high efficiency in solar cells, photodetectors, and light-emitting diodes [1–6]. This is due to their remarkable carrier mobility and diffusion lengths, low exciton binding energies, spectral tunability, and high fluorescence yields [7–10]. HLP typically consists of ABX3 components, where A sites are occupied by monovalent cations such as methylammonium (MA+), formamidinium (FA+), Cs+, and Rb+, divalent Pb2+ cations occupy B sites, and X sites are contributed by one or more types of halide ions such as Cl-, Br-, and I- [11,12]. There are different ways to create high-quality materials for optoelectronic devices in HLP. Some of these methods include molecular beam epitaxy, inkjet printing, and solution-processed spin coating [13–17]. Solution-processed spin coating is a popular and cost-effective approach [18–20]. Various types of lasers, including Fabry-Pérot nanowire lasers, whispering-gallery-mode cavity lasers, and random lasers, have been researched due to their low threshold and high-quality factors [21–23]. For example, MAPbX3 HLP nanowires have displayed low lasing thresholds of 220 nJ/cm2 and high-quality factors of Q∼3600 for room-temperature and wavelength-tunable lasing [24]. Recent research has shown that the integration of CsPbBr3 nanowires with nanostructured indium tin oxide substrates results in ultrahigh quality factor lasing (up to 7860) at room temperature [25]. This is up to four times higher than similar structures on a flat indium tin oxide layer. Additionally, a single crystalline square microdisk of MAPbBr3 prepared using a one-step solution self-assembly method exhibited whispering gallery mode with a quality factor of 430 and a threshold of 3.6 ± 0.5 µJ/cm2 [26]. Other studies have explored the potential applications of cavity-free random lasers in areas such as high-resolution display, biomedical detection, optical storage, and holography [27–29]. Perovskite-based surface-emitting random lasers have also been developed and demonstrated a high-resolution display with speckle-free imaging [30]. Researchers have further worked on selecting the suitable substrate materials and processing microstructures on the HLP and substrates to improve micro-laser performance in HLP [31–35].
Herein, inorganic CsPbBr3 thin films were prepared using the solution-processed spin coating method on the quartz substrates engineered with rectangular arrays using the femtosecond laser ablation technique. Increased absorption spectra were obtained due to the resonant interaction within the engineered structures. Significant enhancement of photoluminescence emission was investigated using a homebuilt confocal microscope. High-performance amplified spontaneous emission (ASE) with typical narrow emission peaks excited using a nanosecond laser centered at 266 nm was obtained with low thresholds. This work provides a convenient and low-cost method for improving and enhancing the performance of optoelectronic devices.
2. Material preparation
To prepare for the growth of inorganic CsPbBr3 thin films, the quartz substrates were thoroughly cleaned with deionized water, acetone, ethanol, and acetone to remove dust and inorganic and organic impurities. The single crystals’ surfaces were then ablated into periodic rectangular arrays using Yb3+: KGW femtosecond lasers at 1030 nm with a frequency of 1.0 kHz and a pulse width of 190 fs. The femtosecond laser light was focused by a lens system to a spot with a radius of 20 µm, as shown in Fig. 1(a), and directed onto the quartz plates on the sample holder in the vacuum chamber. A white light, collimated and overlapped with the femtosecond laser beam, was used to monitor the real-time processing images of the sample surface via a CCD camera. The positions of the samples were controlled by using the horizontal and vertical stepper motors at a speed of 100 µm/s to ensure that the quartz substrate was etched line by line in 5.0 mm × 5.0 mm. The energy densities of the pulse laser were set at 118 W/cm2, 150 W/cm2, 181 W/cm2, and 213 W/cm2, respectively. During the ablation process, high-purity nitrogen gas was pumped into the vacuum chamber as a protection gas. The raw materials of HLP used in this work were anhydrous CsBr (99.999%, Alfa Aesar) and PbBr2 (99.999%, 3A Materials), dissolved in N,N-dimethylformamide (DMF, Alfa Aesar, Purity > 99.8%) at a molar ratio of 1:1. The inorganic CsPbBr3 thin films were grown using the soluble technique prepared in the glove box (with water and oxygen component <0.1 ppm). The CsPbBr3 perovskite thin films were grown at 2500 rounds/min for 60 seconds and annealed at 80 °C for 20 minutes. The CsPbBr3 samples were prepared well when the temperature was cooled to room temperature. To avoid oxidative/hydrolyzing degradation upon exposure to the air atmosphere, the thin films were covered using a quartz glass sheet. The thin films were hermetically encapsulated around them using encapsulating glue, ensuring stable performance of the perovskite thin films during the experimental measurements.
Fig. 1. (a) Schematic of the optical path setup in the femtosecond laser ablation system; Surface morphology of the CsPbBr3 thin films on the ablated quartz substrates with the energy densities of (b) 118 W/cm2, (c) 150 W/cm2, (d) 181 W/cm2, and (e) 213 W/cm2 taken by using the scanning electron microscope (the insets of Fig. (b) to Fig. (e) are the thickness distribution of the ablated structures measured by using the stylus profilometer); (f) Side view of the CsPbBr3 thin films on the quartz substrate ablated at the energy density of 213 W/cm2; The statistical results of (g) the depths, (g) widths and spaces of the grooves along with the etching energy.
3. Experimental results
A scanning electron microscope (SEM) was used to study the surface morphology of CsPbBr3 thin films. Figure 1(b)-1(e) shows that the strip cubes and channels were neatly arranged and alternated with each other. The depth of the channels increased with higher energy density. Due to the high temperature produced by the pulse laser's instantaneous energy, the difference between the channel and strip cube became more noticeable. As seen in Fig. 1(f), the cross-section of the CsPbBr3 perovskite thin films etched under the laser density of 213 W/cm2 was captured using the SEM, and the thickness of the thin film was estimated as 100 nm. The depths of the channels were measured using a stylus profilometer (Veeco Instrument Inc., Dektak 6 M). Figure 1(g) and the insets of Fig. 1(b)-1(e) reveal that the depths of the channels increased from 170 ± 29 nm to 231 ± 33 nm, 348 ± 22 nm, and 480 ± 43 nm with the increase in processing laser energy density. As shown in Fig. 1(h), the widths of the valleys increased from 23.8 ± 2.4 µm to 25.2 ± 3.9 µm, 29.7 ± 4.9 µm, and 30.0 ± 2.0 µm. The interval of the top lands decreased from 26.9 ± 1.3 µm to 24.2± 2.3 µm, 21.1 ± 2.4 µm, and 19.6 ± 2.6 µm.
Using an X-ray diffractor (D/MAX-rB, Rigaku, Japan), the X-ray diffraction patterns of the CsPbBr3 thin films were recorded, as seen in Fig. 2(a), three diffraction peaks centered at 15.4°, 30.6°, and 34.2o were observed, corresponding to the crystallinity of (110), (200), and (210) in the ABX3 perovskite structure. The room temperature absorption spectra of the CsPbBr3 thin films were measured using a UV-Vis-NIR spectrometer (SHIMADZU Inc., UV-2600, Japan), and are presented in Fig. 2(b). An apparent bandgap of around 2.30 eV was observed, and the absorption intensity increased with the ablating energy density. The scattering of light due to the morphological etching and grain boundary/defect formation at ablated surfaces contributed to the pronounced increase in the absorption. Numerically, the optical density of the absorption increased from 0.48 to 3.11 at 520 nm along with the increased etching depth.
Fig. 2. (a) X-ray diffraction patterns of the CsPbBr3 thin film on the ablated quartz substrates; (b) Room temperature absorption spectra of the CsPbBr3 thin film on the ablated quartz substrates.
The distribution and spectra of photoluminescence were examined using a confocal microscope built at home, as displayed in Fig. 3(a). A broad-spectrum white light was directly illuminated onto the sample's surface after passing through a pinhole. Additionally, a violet laser with a central wavelength of 405 nm was focused on the sample after passing through a bandpass filter and a beam splitter (BS). A CCD camera captured the reflected photoluminescence spectra and distributions. Figure 3(b) demonstrates the photoluminescence spectra of CsPbBr3 grown on a flat quartz substrate. The intensity of the photoluminescence emission spectra with a full width at half maximum (FWHM) of 24.4 nm increased as the excitation energy intensity increased. The inset of Fig. 3(b) exhibited uniformly distributed fluorescence emission with some bright spots on the grain boundaries. The photoluminescence emission in the CsPbBr3 thin films on the modified quartz substrates showed a significant enhancement, as depicted in Figs. 3(c)-3(f). It was clearly seen that the stronger the etching energy, the stronger the fluorescence intensity in the films spin-coated on the substrate. Over a 14-fold enhancement was detected in the CsPbBr3 sample that was etched under the power of 213 W/cm2. The insets of Figs. 3(c)-3(f) showed the photoluminescence images on the surface topographies of the samples and captured fluorescence distributions. The fluorescence intensity at the edges was more significant than in other locations due to the scattering effect at the boundaries. Many factors contribute to the enhanced photoluminescence emission: 1) the periodic rectangular arrays modified the light distribution within the arrays since the reflection, diffraction, and scattering interaction of light; 2) irregular boundaries would be formed at the edges of the periodic rectangular arrays since the thermal effect of the femtosecond laser ablation process, which increased the scattering and absorption of light; 3) it works as a waveguide since the diffractive index difference between the perovskite/quartz arrays and air, where the fluorescence light intensity is more concentrated in the waveguide structure, as shown in Fig. 3(c) to 3(e).
Fig. 3. (a) Schematic of the optical path of the home-built confocal fluorescence microscope; (b) The photoluminescence emission spectra of the CsPbBr3 thin films on flat quartz substrate; The photoluminescence emission spectra of the CsPbBr3 thin films on the etched quartz substrate under the energy power of (c) 118W/cm2, (d) 150 W/cm2, (e) 181 W/cm2, and (f) 213 W/cm2. (The insets of (b)-(f) were the fluorescence images on the surface of the thin films.); (g) The peak intensity of the photoluminescence emission spectra changes along with the excitation energy of the laser at 405 nm.
To study the performance of the ASE in the CsPbBr3 thin films on a patterned quartz substrate, a nanosecond laser centered at 266 nm with a pulse frequency of 10 Hz and a pulse width of 8 ns (Changchun New Industry Inc., DPS-266-Q) was used as the excitation source and focused on the surface of the perovskite thin films with a diameter of 2.0 mm. As seen in Fig. 4(a), broadband photoluminescence spectra centered at 534.7 nm could be observed in CsPbBr3 thin film on a flat quartz substrate with an FWHM of 23.1 nm, and the intensity of the fluorescence emission increased with the excitation energy. However, no spectral narrowing and ASE peaks appeared. In the etched quartz substrate sample engineered under the laser energy density of 118 W/cm2, as seen in Fig. 4(b), only broadband photoluminescence could be observed with an FWHM of 27.5 nm when the excitation energy was weak. The FWHM reduced to 2.6 nm rapidly, and the sharp lasing emission could be observed above the threshold, which increased dramatically with the excitation energy density. A remarkable transition threshold from photoluminescence to ASE around 1.38 mJ/cm2 could be seen. Lower thresholds and more intensive ASE were achieved in the samples with higher etching energies of the femtosecond lasers, as seen in Fig. 4(c)-4(e). The bi-Gaussian model was used to analyze the emission area of the photoluminescence and ASE intensity, as seen in Fig. 4(f) and 4(g). The emission intensities increased dramatically with excitation energy, and the ASE was more intensive than the photoluminescence emission in the sample etched under the laser energy density of 213 W/cm2. It showed nearly 10-fold enhancement in both photoluminescence and ASE. When the laser beam incident onto the surface of the periodic arrays, light was scattered and diffracted within the periodic structure. The deeper the arrays are, the larger of the effective thickness of the sample and the perovskite thin films would be [36,37], which increases the efficiency of the ASE excitation and reduces the thresholds of the ASE. As seen in Fig. 4(h), the sharp ASE emission appeared at the thresholds of 1.38 mJ/cm2, 520 µJ/cm2, 490 µJ/cm2, and 410 µJ/cm2, respectively. Since the laser used in this work is a low frequency (10 Hz) nanosecond laser, the average threshold intensities of the ASE are 13.8 mW/cm2, 5.2 mW/cm2, 4.9 mW/cm2, 4.1 mW/cm2, which are comparable or lower than the thresholds of ASE and lasing in other works [24–26].
Fig. 4. (a) Photoluminescence emission spectra of the CsPbBr3 thin films on flat quartz substrate excited by using the nanosecond laser centered at 266 nm; Amplified spontaneous emission spectra of the CsPbBr3 thin films on the etched substrates under the femtosecond laser intensity of (b) 118 W/cm2, (c)150 W/cm2, (d)181 W/cm2, and (e)150 W/cm2; The emission area intensity of the peak centered at (f) 525 nm and (g) 545 nm of the photoluminescence emission and ASE emission spectra in the CsPbBr3 thin films on the etched substrates under the femtosecond laser intensity of 118 W/cm2, 150 W/cm2, 181 W/cm2, and 150 W/cm2; (h) The FWHM of the ASE emission spectra in the CsPbBr3 thin films on the etched substrates under the femtosecond laser intensity of 118 W/cm2, 150 W/cm2, 181 W/cm2, and 150 W/cm2.
To further explore the optical properties of the CsPbBr3 thin films on flat and etched quartz substrates, the optical gains of the CsPbBr3 thin films on the flat and the etched substrates were measured using the variable stripe lengths (VSL) method [38,39]. As seen in Fig. 5(a), the nanosecond laser centered at 266 nm was first focused into an inline laser beam using the cylindrical lens, which illuminated the surface of the samples. The stripe length of the laser beam was controlled by using a tunable slit with a minimum step scale of 10.0 µm. The photoluminescence and ASE were measured on one edge vertically to the laser beam, which increased rapidly with the width of the slit. As seen in Fig. 5(b), the intensity of the ASE increased dramatically with the increase of the slit width, and the optical gain coefficient of the thin film could be calculated by fitting the data with the formula ${I_0}(L )= {I_s}\frac{A}{g}[{exp({gL} )- 1} ]$ [40]. Where g is the optical gain coefficient of the material, A is the area of the laser spot, L is the width of the variable slit, and Is is the emissivity of the ASE. As seen in Fig. 5(c), the optical gain value of the CsPbBr3 thin films on the flat substrate was 25.4 ± 2.9 cm-1. The value increased dramatically to 391.0 ± 30.0 cm-1 in the samples on the etched substrates along with the etching power of the femtosecond laser, which provided an additional dimensional to improve the optical properties of the laser materials. As discussed above, the deeper the arrays are, the larger the adequate thickness of the sample and the perovskite thin films would be. The optical path length of the laser beam would be effectively increased due to the waveguide effect and the multiple scattering of light. The efficiency of the ASE excitation could be increased, and the gain coefficient of the perovskite thin films would also increase.
Fig. 5. (a) The schematic setup of the optical gains of the CsPbBr3 thin films using the variable stripe length (VSL) method; (b) The optical gains curves of the CsPbBr3 thin films on the flat and etched substrates measured by using the VSL method; (c) The statistical value of the gain coefficient changes along with the etching energy of the femtosecond laser.
4. Conclusion
In conclusion, the femtosecond laser ablation technique was used to enhance photoluminescence emission in CsPbBr3 thin films by etching the quartz substrate with periodic rectangular arrays where the peak-valley heights, widths, and spaces were controlled by using different excitation energy. The increased peak-valley heights enhanced the adequate thickness of light wave propagation in the sample, which in turn efficiently enhanced the photoluminescence of the thin films. The optical path length of the laser beam effectively increased because of the waveguide effect and the multiple scattering of light, and the thresholds of the ASE reduced from 1.38 mJ/cm2 to 410 µJ/cm2, which is very low in ASEs and lasers excited with low-frequency nanosecond lasers. As high as 15-fold enhancement of the optical gain coefficient was achieved using the VSL measurement, providing another dimension to increase the performance of the lasing materials. Although the femtosecond laser ablation technique has limitations in fine processing and material selections due to its thermal inhomogeneity, it provides a convenient and feasible way to improve the performance of fluorescence and lasing emission. This is highly needed and significant in developing efficient light sources, electro-optical devices, and lasers.
Funding
National Natural Science Foundation of China (11804036); Natural Science Foundation of Chongqing (CSTB2023NSCQ-MSX0807); the Experimental Technology Research Project of Southwest University (SYJ2023035); Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN202200521); Science and Technology Innovation Team of Guizhou Education Department ([2023]094).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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